Vidicon camera circuit



United States Patent US. Cl. 315-30 4 Claims ABSTRACT OF THE DISCLOSUREA circuit is provided to stabilize beam current in a vidicon cameratube. A vidicon tube has an electron emitting cathode, a controlelectrode, and a light responsive signal electrode. Conventional meanscollect most of the electrons emitted by said cathode and focus arelatively small portion of the electrons emitted by the cathode into abeam. Conventional means cause the beam to scan the signal electrode toproduce a picture signal related to the distribution of light on thesignal electrode. The stabilizing circuit comprises a resistor of largeresistance connected between the cathode and a source of fixed referencepotential. The control electrode is preferably grounded. The electroncurrent from the cathode produces a voltage across the resistor whichadds to the reference potential to bias the cathode relative to thecontrol electrode to produce electron current to the signal electrode ofsuitable intensity, the resistance of the resistor being so large thatthe resistor responds to changes in the electron current from thecathode by producing a corresponding change in bias on the cathodesufiicient to maintain the average flow of electrons from the cathodesubstantially constant. Positive blanking pulses are applied to thecathode to cut off the flow of electrons during the pulses.

This invention relates to television cameras and more particularly tothe control of beam current striking the signal electrode of a vidicontube. Still more particularly, the invention relates to establishing anelectron beam of suitable intensity in a vidicon tube and to stabilizingthis beam intensity with aging of the tube.

Television cameras commonly include a vidicon tube having an electronemitting cathode, a control electrode, various focusing electrodes and alight-responsive signal electrode. In cameras using vidicon tubes,electrons are emitted from the cathode at a rate dependent upon therelative potential of the adjacent control electrode, as well as uponthe temperature and emission qualities of the cathode. By means of thefocusing electrodes, in association with a focusing magnetic field,certain of the emitted electrons are formed into a relatively sharp beamdirected toward the signal electrode. Horizontal and vertical deflectioncoils adjacent the tube deflect the electron beam to cause it to scanthe signal electrode. In the course of the scanning, the electrons inthe beam discharge the charge formed on the signal electrode by theaction of incident light and thereupon produce an output signal relatedto the distribution of the incident light upon the signal electrode.This output signal is then processed into a composite picture signalwhich may be utilized for transmission of a television signal or foroperation of a video recorder or monitor.

In order to develop a proper output signal, it is necessary to scan thesignal electrode with a beam of suitable intensity to deposit thedesired charge upon the signal electrode. The beam current, beingdependent upon the difl'erence in voltage between the cathode and thecontrol electrode, can be controlled by this difference. The controlelectrode must be biased negatively relative to the "ice cathode inorder to establish the desired current. To this end, it is conventionalto ground the cathode and to apply a negative bias potential to thecontrol electrode.

In the scanning operation in television cameras, the beam is deflectedfrom one side to the other and from top to bottom, and it is necessaryto blank the beam current, that is, to turn it ofl, during the retraceof the scanning operation, otherwise the retrace will appear on thereproduced picture. Blanking pulses derived from the deflection circuitsare used for this purpose. Often, a resistor is inserted between thecathode and ground, and positive pulses are applied to the cathode toturn ofl the beam current during the retrace.

As vidicon tubes age, the cathode electron emissivity ordinarilydecreases, thus requiring lower bias to achieve the desired beamcurrent. To achieve this lower bias, the negative power supply coupledto the control electrode is ordinarily made variable so that adjustmentscan be made from time to time during the life of the tube.

According to the present invention, the negative power supply and itscontrol are both eliminated. Instead, a relatively large cathoderesistor is used to develop the required bias. The resistor may bereturned to a positive potential. There is then no need for any negativepower supply, and the control electrode can be grounded, thus omitting anumber of costly parts.

Vidicon tubes are inherently ineflicient. Most of the electrons emittedfrom the cathode travel to the focusing electrodes without ever reachingthe signal electrode as part of the directed beam. Ordinarily, thismerely represents wasted current and power without serving a usefulpurpose. The present invention, however, takes advantage of the factthat the signal electrode current is negligible in magnitude relative tothe magnitude of the current flowing to the focusing electrodes. Itpermits a relatively large cathode resistor to be introduced without anysubstantial degeneration of the signal. That is, the total electronemission current is so large relative to the signal electrode currentthat variations in the signal electrode current have substantially noeflect upon the cathode potential. At the same time, the relativelylarge current flowing to the focusing electrodes has a material effectupon the cathode potential.

As the emissivity of the cathode begins to drop off with the aging ofthe tube, the current to the focusing electrodes decreases. The flow ofcurrent through the cathode resistor therefore decreases. The voltageacross the cathode resistor therefore decreases, thus reducing thepotential of the cathode relative to the control electrode. Thisrepresents a decrease in bias which in turn tends to increase the flowof electrons from the cathode to balance the decrease occasioned by lossof emissivity, hence stabilizing the cathode current.

It is therefore a primary object of the present invention to control thebeam current in a vidicon tube. It is a further object of the presentinvention to stabilize the beam current in a vidicon tube upon the agingof the tube. It is still a further object of the invention to controlbeam current without the need for negative power supplies. Anotherobject of the invention is to control beam current in a vidicon tubeoperated with its control electrode grounded and without the need for anegative power supply.

Further objects and advantages of the present invention will becomeapparent from consideration of the following description, particularlywhen taken in connection with the appended drawing in which:

The single figure is a diagrammatic illustration of a preferred form ofthe beam control apparatus of the present invention.

The drawing illustrates a television camera circuit and in particularone including a vidicon tube 10. The vidicon tube includes a cathode l2indirectly heated by a filament 14, the filament 14 being connected by afilament power supply 16 which supplies the power to heat the filament.The tube further contains a control electrode 18 and some sort offocusing electrode means illustrated generally as focusing electrodes 20anad 22. In some vidicon tubes, there may be additional focusingelectrodes. Vidicon tubes generally include as focusing electrodes aplurality of cylindrical electrodes having suitable apertures fordefining and focusing an electron beam. The tube further includes asignal electrode 24. Associated with the vidicon tube are horizontal andvertical deflecting coils 26 and a focusing coil 28.

In accordance with the preferred embodiment of the present invention,the control electrode 18 is directly connected to ground. The cathode 12is connected to one end of a resistor 30 the other end of which isconnected to the positive side of a low voltage regulated power supply32. A high voltage regulated power supply 34 supplies positive potentialthrough a load resistor 36 to the signal electrode 24. The focusingelectrode may be connected to the same terminal of the power supply 34.Further, this power supply 34 may supply positive potential to focusingcoil 22 through a voltage divider 37 comprising resistors 38 and 39.

The operation of the vidicon tube may be explained as follows: Theheated filament 14 heats the cathode 12 causing it to emit electrons.The emitted electrons are attracted to the focusing electrodes 20 and 22and to the signal electrode 24 because of the high positive potentialapplied to these electrodes. A more positive potential is applied tothese electrodes than appears on the cathode 12. The magnitude of thiselectron current is limited, however, by the relative potential of thecontrol electrode 18. The control electrode 18 is at a potentialnegative with respect to the potential of the cathode 12 and thereforeinhibits the free flow of electrons.

Electrons from the cathode that pass the control electrode 18 aredefined into a narrow beam by the conjoint operation of the focusingelectrodes 20 and 22 and the focusing coil 28. The focusing coil 28 isenergized by means not shown to provide an axial magnetic field formagnetic focusing of the moving electrons. The focusing electrodes 20and 22 provide electrostatic focusing which operates in conjunction withthe magnetic focusing to produce a relatively narrow beam of electrons.The horizontal and vertical deflecting coils 26 supply additionalmagnetic fields which cause the electron beam to be deflectedmagnetically, causing the beam to scan the surface of the signalelectrode 24 in accordance with the conventional television scanningscheme. The fields produced by the horizontal and vertical deflectingcoils are controlled by respective signals from a horizontal deflectioncircuit 40 and a vertical deflection circuit 42.

The signal electrode 24 is photosensitive. The action of the light uponthe signal electrode builds up a charge pattern upon the electrode. Itis the discharge of this charge pattern by the electron beam thatproduces the flow of current through the load resistor 36 therebydeveloping an output signal at the signal electrode 24. This signal isapplied through a coupling capacitor 44 to an amplifier 46 which may, asshown, comprise a number of stages of amplification. The output of thisamplifier, with appropriate signal processing to include synchronizingpulses, becomes the composite picture signal which may be used tomodulate a carrier signal for television transmission or may be used tooperate a monitor or a video recorder.

The horizontal and vertical deflection circuits 40, 42 develop signalswhich may be used for developing appropriate blanking and synchronizingpulses. These signals are applied to a blanking pulse circuit 48 todevelop horizontal and vertical blanking pulses. These blanking pulsesare applied through a coupling capacitor 49 to the cathode 12 to stopelectron emission during the time of the retracing of the electron beamafter each horizontal and vertical sweep. This is necessary to avoiddestroying any of the charge on the signal electrode during retracewhich would adversely affect the reproduced picture. At the same timethe blanking pulses are applied to one of the stages of the amplifier 46to form the blanking pulses in the composite picture signal. I

The signals from the deflection circuits 40, 42 are also applied to asynchronizing pulse circuit 50 to develop horizontal and verticalsynchronizing pulses. These pulses are applied to a succeeding stage ofthe amplifier 46 to form the horizontal and vertical synchronizingpulses in the composite picture signal.

A vidicon tube typically operates with the electrode 20 at about 300volts positive and the electrode 22 at a slightly lower positivevoltage. With about 300 volts positive applied to the load resistor 36,the signal electrode 24 operates in the range of 50 volts positive, whenbeam current is flowing to the signal electrode 24. Typical bias betweenthe cathode 12 and the control electrode 18 is about 40 volts.

In accordance with the present invention, this cathode to controlelectrode bias is achieved by the conjoint operation of the power supply32 and cathode current flowing through the resistor 30. Typically, thepower supply 32 is a regulated power supply developing about 18 voltspositive. Typical cathode emission current is about 500 microamperes,most of which is collected by the focusing electrodes 20 and 22. Theportion of the current in the electron beam that strikes the signalelectrode is typically less than one-tenth of one percent of the totalcathode current. With a cathode current of 500 microamperes, a cathoderesistor 30 of 47,000 ohms resistance develops a voltage thereacross of23.5 volts, thus making the bias between the cathode 12 and the controlgrid 18 about 41.5 volts.

The blanking pulses are formed in a blanking pulse circuit 48 andapplied through a capacitor 49 to the cathode 12. These blanking pulsesare typically 18 volts positive and serve to cut off the cathode currentfor the duration of the pulse. The resistor 30 isolates these pulsesfrom ground.

The various vidicon tubes differ in their characteristics. It istherefore often desirable that the resistance of the resistor 30 bedetermined empirically. To so determine the proper resistance, thevidicon tube may be placed in operation and the resultant pictureobserved upon a monitor. Since the beam current striking the signalelectrode determines the highlights in the monitored picture, theresistor 30 may then be adjusted to establish the beam current giving asuitable picture. Adjustment of the resistor 30 adjusts the cathode tocontrol electrode bias which controls the total electron emission fromthe cathode and hence that part which forms the electron beam. Theresistor 30 may therefore be adjusted to that resistance which producesproper discharge of picture highlights. Once the proper resistance isestablished, a fixed resistor may be used as resistor 30.

The use of cathode resistors to develop cathode to control electrodebias is, of course, old; however, when cathode resistors are used todevelop such bias, they are bypassed by a capacitor in order not todegenerate the sig nal. That is, when the signal current flows throughthe cathode resistor, it ordinarily creates a bias which develops anopposing signal degenerating the signal voltage. In the present circuitit is undesirable to bypass the resistor 30 with a capacitor, for it isdesired to apply blanking pulses to the cathode, and a bypassingcapacitor would bypass the blanking pulses.

The present invention not only operates without bypassing the resistor,but takes advantage of the great inefficiency of the vidicon tube todevelop the desired bias without being degenerative in respect to thesignal. As noted earlier, the signal electrode current is typically lessthan one-tenth of one percent of the total cathode current. Most of thecathode current is wasted in being collected by the focusing electrodes20 and 22. Because of this waste, however, the cathode current can beused to develop the cathode to control electrode bias withoutdegeneration of the signal. That is, the very large cathode current canbe stabilized at about 500 microamperes without any substantial effectupon the signal current portion.

If the cathode current decreases, as for example be--' cause of aging ofthe tube, the voltage developed across the resistor 30 decreasescorrespondingly. This reduces the cathode to control grid bias, thusincreasing the electron flow and tending to restore the cathode current.At the same time the signal current portion varies over only a fewtenths of a microampere. Such variations have a negligible etfect uponthe voltage developed across the resistor 30. Thus, although the circuitis highly degenerative in respect to the total electron current and thefocusing electrode current, it is not degenerative in respect to thesignal current.

Although a preferred embodiment has been described in some detail,various modifications thereof can be made without departing from thescope of this invention. For example, under some circumstances the lowvoltage power supply 32 can be eliminated and the resistor 30 connecteddirectly to ground; under these circumstances the entire cathode tocontrol grid bias can be developed by the cathode current flowingthrough the resistor 30.

Other modifications are also within the scope of the invention asclaimed.

I claim:

1. In a vidicon camera including a vidicon tube having an electronemitting cathode, a control electrode, and a light responsive signalelectrode, means for collecting most of the electrons emitted by saidcathode and focusing a relatively small portion of the electrons emittedby said cathode into a beam, means for scanning said signal electrodewith said beam to produce an output signal related to the distributionof light on said signal electrode,

and means for applying positive blanking pulses to said cathode to cutoff the flow of electrons during said pulses: circuit means connected tosaid cathode for stabilizing the emission of electrons from saidcathode, said circuit means comprising a resistor of large resistanceconnected between said cathode and a source of first fixed referencepotential, means for connecting said control electrode to a source ofsecond fixed reference potential, the electron current from said cathodeproducing a voltage across said resistor which adds to said firstreference potential to bias said cathode relative to said controlelectrode to produce electron current to said signal electrode ofsuitable intensity, the resistance of said resistor being a substantialfraction of the effective resistance traversed by said electron currentso that said resistor responds to changes in said electron current fromsaid cathode by producing a corresponding change in bias on said cathodesufficient to maintain the average flow of electrons from said cathodesubstantially constant.

2. A circuit according to claim 1 wherein said second referencepotential is ground and said first reference potential is at least aspositive as ground.

3. A circuit according to claim 1 wherein said resistor is unbypassed.

4. A circuit according to claim 1 wherein said first and secondreference potentials are substantially the same.

References Cited UNITED STATES PATENTS 2,911,561 11/1959 Fathauer 315-30X 3,168,679 2/1965 Kobbe 315--30 2,911,562 11/1959 Fathauer 1787.2

RODNEY D. BENNETT, Primary Examiner. C. E. WANDS, Assistant Examiner.

